The digitization of a high bandwidth analog signal having a large dynamic range requires analog to digital conversion means having a correspondingly high dynamic range capability. Other performance characteristics include the accuracy of the converter, which is defined in absolute terms as the number of bits to which the output signal correlates with the "ideal" digital representation of the analog input signal; and resolution, which is commonly expressed as the number of bits M where the converter has 2.sup.M possible states. The highest performance monolithic conversion devices have difficulty providing better than 10 bit resolution at 75 million samples/second of high dynamic range signals.
The coding requirements for a telecine (a machine for converting film images into video signals) are more stringent than for other video systems, e.g., a video camera. Telecines that digitize the video signal directly have especially stringent A/D converter resolution and accuracy requirements, because the film scanned by the telecine has particular characteristics that need to be compensated in order to provide good television pictures. In particular, the telecine must process a much larger input contrast range than a live camera because of the expansion in contrast produced by the film gamma. If the digitization of the primary R, G, B signals take place before gamma correction, at least 11 bits are ordinarily needed for a telecine video processing channel.
Various methods have been used to achieve a large dynamic range capability in the high-resolution analog-to-digital circuitry of a video system. Four typical ones include the use of a signal compandor, such as a logarithmic amplifier upstream of the A/D converter; parallel conversion; sub-ranging multiple converters; and level-dependant (dual-range or range-changing) converter architectures.
Signal companding A/D converter circuits have a signal dependent bandwidth and an unsymmetrical pulse response. The matching of the companding function in multiple channel systems (Red, Green, Blue) is also problematic. High speed, single-stage A/D converters in the form of parallel, or flash, conversion stages, and multi-state A/D converters (e.g., subranging converters) are still very costly and complex to implement in high dynamic range, high-resolution configurations.
With respect to signals that have been linearly coded and subsequently gamma corrected, tests of visibility of the quantization effects indicate that the smallest perceived fractional change in luminance is about 2%, and the perceived fractional change does not rise rapidly above 2% until the signal level falls to below about 15%. A preamplification factor of 8 can be applied to input signals falling below 12.5% of peak (the predetermined threshold) to add three bits at signal levels below 12.5% (see "A Digital Telecine Processing Channel," by A. Oliphant and M. Weston, SMPTE Journal, July 1979, vol. 88, pp. 474-480).
For example, the model B3410 Telecine, manufactured by Marconi Communications Systems Ltd., England, provides 11 bit A/D conversion by use of an 8 bit A/D converter and a gain switching system. The A/D conversion has two ranges: a normal accuracy range and a fine accuracy range. Input signals falling between 12.5% and 100% of peak white are digitized over the whole 8 bit range of the converter to provide eight MSBs of the output signal. Input signals falling below 12.5% of peak are amplified eight times and then digitized to provide eight LSBs of the 11 bit output signal. (See "Digital Video Processing for Telecine," by R. Matchell, IBC 1981, IEE Conference Publication N110, IEE London, U.K., pgs. 41-45; "The Marconi B3410 Line Array Telecine," by R. Matchell, SMPTE Journal. November 1982, pp. 1066-1070).
Hence, to achieve greater resolution without the greatly increased complexity of developing an extended range A/D converter, certain converter circuits have been devised wherein low-cost converters (e.g. 8-bit A/D converters) are modified so as to insert analog preamplification whenever the input signal falls below a predetermined threshold. The preamplification factor is ordinarily an exact power of 2; thus the eight-bit word from the A/D converter can be located within a longer word by simply displacing it by the appropriate number of binary places.
Such level-dependent A/D converters are sometimes called dual range (or range-changing) converters, and typically include two separate conversion paths wherein one path has gain equal to a multiple of the gain of the other path. A comparator, or a comparison-type operation, switches the input samples from one path to the other as the input video signal level passes a preset threshold level. One approach is to "gang" together two flash A/D converters such that the low gain path in effect has a coarser step size than the high gain path (see the dual-ranging A/D converter disclosed in "High-resolution digitization of photographic images with an area charge-coupled device (CCD) imager" by J. R. Milch, SPIE Vol. 697, Applications of Digital Image Processing IX (1986), pp. 96-104).
A known dual range A/D converter of this type is shown in FIG. 1. Input analog signals are applied to a first M bit A/D converter 10 that is used in a low gain signal path A to generate first digital signals and a second M bit A/D converter 12 that is used in a high gain signal path B to generate second digital signals. The low gain path A is provided with unity gain from an amplifier 14 while the high gain path B is provided with a gain of 2**N from an amplifier 16. The low gain path is used when the input signal V.sub.IN is greater than (Vmax/2**N) and the high gain path is used when the input signal is less than or equal to (Vmax/2**N). (Vmax is the maximum value that signal V.sub.IN may become.)
A comparator 18 compares the code values of the second digital signals to a suitable switch-over code word (usually just less than Vmax/2**N) and thereby controls a digital multiplexer 20 to select the data path to be utilized. (An overflow flag (OVF) from the second A/D converter 12 could also be used to control the multiplexer 20. Similarly, an analog comparator could be used to compare the analog input to the A/D converters 10 and 12.)
The multiplexer 20 includes registers 20a and 20b for assembling an output code word from the M data bits and N zero bits, the latter being joined to the M data bits to fill out the output code word. The output of the multiplexer 20 thus is (M+N) bits, wherein the N bits form the zero LSBs of the low gain signal path A and the highest MSBs of the high gain signal path B. In a typical application, the M bit A/D converters 10 and 12 are conventional 9 bit (M=9) flash-type A/D converters, and a gain of eight (i.e., N=3) is applied to the high gain path B, thereby resulting in an output of 12(M+N) bits from the multiplexer 20. This results in two accuracy ranges; high accuracy at low signal levels and low accuracy at high signal levels (see FIG. 2). (The added accuracy N, is determined by the log base 2 of the gain in the amplifier prior to the lower A/D converter 12, while the base accuracy M is determined by the individual A/D converter. The resultant data word size is (M+N) bits.) This can be thought of as a crude approximation to the characteristics of the human visual system, and provides a near constant (quantization step size to input signal level) ratio after digitization.
The signal paths of the dual range A/D converter shown in FIG. 1 have an equivalent digital accuracy (analog signal versus A/D conversion accuracy) for each individual A/D converter as illustrated in FIG. 2.
Techniques such as adding analog dither to the image signal, appending digital noise with an appropriate probability density function (PDF) to the lower order bits, and the use of "Roberts Noise" (Roberts, L.G., "Picture Coding Using Pseudo-Random Noise", IRE Transactions On Information Theory. February 1962) have been commonly used in imaging systems to break up contouring effects and reduce quantization visibility. Although these methods reduce the number of bits required by the A/D converter, they do not address the above-described dynamic range requirements of the telecine and other video systems having stringent dynamic range requirements.